April 22, 2014 | Leave a Comment
Northwestern University (NU) scientists have discovered a surprising material that is the best in the world at converting waste heat to useful electricity. An interdisciplinary team led by inorganic chemist Mercouri G. Kanatzidis found the crystal form of the chemical compound tin selenide conducts heat so poorly through its lattice structure that it is the most efficient thermoelectric material known.
The world’s energy problem could be reduced by stopping the wasting of so much energy when producing and using it. Thermal losses happen in very high proportions in power generating plants and transportation by cars, trucks, and planes where about two-thirds of the energy input is simply lost as waste heat.
The NU material has an outstanding property that could be exploited in solid-state thermoelectric devices in a variety of industries, with potentially enormous energy savings. Unlike most thermoelectric materials, tin selenide has a simple structure, much like that of an accordion, which provides the key to its exceptional properties.
Thermoelectrics are built on thin blocks of semiconductor with a useful property: heating them on one side generates an electric voltage that can be used to drive a current and power devices. To obtain that voltage, thermoelectrics must be good electrical conductors but poor conductors of heat.
The efficiency of waste heat conversion in thermoelectrics is reflected by its figure of merit, called ZT. Tin selenide exhibits a ZT of 2.6, the highest reported to date at around 650º Celsius. The material’s extremely low thermal conductivity boosts the ZT to this high level, while still retaining good electrical conductivity.
The ZT metric represents a ratio of electrical conductivity and thermoelectric power in the numerator (which needs to be high) and thermal conductivity in the denominator (which needs to be low).
Potential areas of application for the high-temperature thermoelectric material include the automobile industry (a significant amount of gasoline’s potential energy goes out of a vehicle’s tailpipe), heavy manufacturing industries (such as glass and brick making, refineries, coal- and gas-fired power plants) and places where large combustion engines operate continuously (such as in large ships and tankers).
Vinayak P. Dravid, a senior researcher on the team said, “A good thermoelectric material is a business proposition — as much commercial as it is scientific. You don’t have to convert much of the world’s wasted energy into useful energy to make a material very exciting. We need a portfolio of solutions to the energy problem, and thermoelectric materials can play an important role.”
Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry in the Weinberg College of Arts and Sciences said, “The inefficiency of current thermoelectric materials has limited their commercial use. We expect a tin selenide system implemented in thermoelectric devices to be more efficient than other systems in converting waste heat to useful electricity.”
Two years have passed since the same research group broke the world record with another thermoelectric material they developed in the lab with a ZT of 2.2. The material, despite having a very simple structure, conducts heat so poorly that even moderate thermoelectric power and electrical conductivity are enough to provide high thermoelectric performance at high temperature.
The researchers did not expect to find tin selenide to be such a good thermoelectric material.
Lidong Zhao, a postdoctoral fellow in Kanatzidis’ research group, grew crystals of tin selenide and measured the crystal in three directions, along each axis. He found that the thermal conductivity was “ridiculously low” along the a-axis but also along the other two axes.
Dravid said, “The results are eye-opening because they point in a direction others would not look. This material has the potential to be applied to other areas, such as thermal barrier coatings.”
Those properties gave the material a ZT of 2.6, the best value ever measured. The key to the ultralow thermal conductivity, Kanatzidis says, appears to be the pleated arrangement of tin and selenium atoms in the material, which looks like an accordion. The pattern seems to help the atoms flex when hit by heat-transmitting vibrations called phonons, thus dampening SbSe’s ability to conduct heat.
The threshold for practical marketable thermoelectrics looks to be a ZT of 3. The new NU material offers lessons on how to get there.